An intense interest has arisen in fermentation and gasification of carbohydrate-rich biomass to provide alternatives to petrochemical sources for fuels, livestock feeds and for organic chemical precursors.
Lignocellulosic biomasses including industrial and agricultural wastes have proved particularly interesting as carbohydrate sources.
For lignocellulosic biomasses subject to processing by enzymatic hydrolysis, pretreatment is generally required in order to separate lignin and hemicellulose from cellulose fibers and thereby increase catalytically effective access of hydrolytic enzymes. A variety of pretreatment processes have been reported, many of which rely on high temperature treatments at high pressures. For review, see ref. 1 and 2.
Discharge of pretreated biomasses from high pressure regions to subsequent processing at lower or atmospheric pressure presents a technical problem that has previously been solved by three general approaches. Sluice systems suitable for semi-continuous processing have been developed that provide pressure sealed transfer of biomass from regions of different pressure. See for example, WO 2011/024145, which is hereby incorporated by reference in entirety.
Alternatively, discharge of pretreated biomass from high pressure regions has been achieved using “steam explosion,” where biomass is literally blown out of a pressure treatment device into a blow tank. See for example U.S. Pat. No. 6,506,282. Or alternatively still, biomass can be discharged from pressurized pretreatment reactors by means of a “hydrocyclone” system, such as that described in WO 2009/147512 A2, which is hereby incorporated by reference in entirety.
WO 2009/147512 A2 discloses a device for discharging pretreated biomass from higher to lower pressure regions. Pretreated, hot biomass is added under high pressure at the top of a discharge vessel, which is partially filled with water or aqueous solution. The vessel comprises a plurality of water jets situated on its sides through which water or aqueous solution is injected in order to establish a strong turbulence in the lower part of the vessel.
This turbulence, preferably a vortex turbulence or “hydrocyclone”, blends biomass and water prior to its ejection at high speed as a slurry through an orifice or valve at the bottom of the vessel. A stable temperature stratification is established within the discharge vessel, whereby hot water remains at the top of the vessel, reducing heat and steam loss from the pretreatment reactor.
Within the pulp and paper industry it is well known to use a pulper for disintegration of waste paper fibres. Such a pulper comprises a vessel comprising in a lower part a rotator or agitator in the form of a conical screw which imparts physical impact to the waste paper thereby resulting in a disintegration of the waste paper fibres. During use of such a paper pulper process water is added to the waste paper. An example of such a pulper for use in the waste paper industry is disclosed in U.S. Pat. No. 4,460,132. The apparatus disclosed in U.S. Pat. No. 4,460,132 comprises no means for being connected with a pressurized biomass pretreatment device and is therefore not suitable for use for discharging pretreated biomass from higher to lower pressures.
Another apparatus for use in the paper and pulp industry is disclosed in U.S. Pat. No. 4,725,295. U.S. Pat. No. 4,725,295 discloses an apparatus for separating solid particulates from a gaseous fluid in pressurized processing system for use within the paper pulping industry. The apparatus comprises a steaming tube having inlets at its top for wood chips and steam. In the lower part of the steaming tube is arranged a rotary agitator. Also arranged at the lower part of the steaming tube is a discharge for wood chips which have not been made into pulp. Downstream of the steaming tube the manufactured pulp passes a thermo-mechanical refiner, whereafter the pulp, driven by pressure, enters a cyclone. The cyclone itself comprises an upper part comprising a steam exhaust outlet and a lower part comprising an impeller. The impeller is designed for sweeping out paper pulp via a pulp outlet orifice using the sweep orifice principle. As the wood chips in the process is mainly treated with steam with essentially no liquid water being present, the problems of depressurizing an aqueous slurry of a biomass comprising considerably amounts of liquid water being present at super atmospheric pressures are not encountered in the invention of U.S. Pat. No. 4,725,295. The apparatus disclosed in U.S. Pat. No. 4,725,295 does not provide any means allowing safe transfer of an aqueous slurry of a biomass comprising considerably amounts of liquid water being present at super atmospheric pressures from this super atmospheric region to a low pressure region.
Other systems involving combinations of agitators in the processing of slurries or fluids are disclosed in WO 2009/137867 A1, WO 03/045525 A1 and U.S. Pat. No. 6,428,591 B1, respectively.
WO 2009/137867 A1 discloses an apparatus for deaerating a feed liquid comprising a liquid suspension or pulp. The apparatus comprises a feed conduit to convey a feed liquid into a separator. The separator comprising a mechanical agitator for inducing a rotational flow of the feed liquid in a separation chamber such that the rotational flow generates a centrifugal vortex to separate the feed liquid into a first component consisting essentially of froth or gas and a second component consisting essentially of deaerated liquid or sludge. A lower output channel allows for conducting the second component out of the separator. An overflow channel in the form of an outlet for froth is arranged in an upper part of the separator, hence providing direct access from the interior of the separator to a point outside the separator. Due to the presence of this overflow channel, the separator disclosed in WO2009/137867 A1 will not be suitable for processing sludge being present at superatmospheric pressures.
WO 03/045525 A1 discloses a vortex separator for separating materials into three fractions having different specific gravity. The separator comprises a vortex tube having an upper end and a lower end. The upper end comprises a feed inlet for introducing material into the separator. The lower end of the separator comprises a lowest outlet opening arranged at an inner location of the separator and adapted to collect the materials having the highest specific gravity; a highest outlet opening arranged at an outer location of the separator and adapted to collect the materials having the lowest specific gravity; and a middle outlet opening arranged at a middle location of the separator and adapted to collect the materials having a medium specific gravity. An impeller for creating a vortex of the materials being fed to the separator is arranged at top of the separator. Due to the arrangement of the stirrer at the top, this apparatus will not be suitable for providing stirring of a slurry of a solid material entering the apparatus and specifically this apparatus will not be suitable for transferring pretreated biomass from higher to lower pressure regions.
U.S. Pat. No. 6,428,591 B1 discloses a cyclone for use in a pressurized materials processing system, such as a medium density fiberboard (MDF) processing system, for separating solid material which is entrained in a gaseous fluid maintained at elevated pressure and for reducing the emission of VOCs. The cyclone comprises a housing having an upper end, a lower end and a substantially cylindrical side wall defining a longitudinal axis. An inlet is formed within the housing proximate to the upper end and is tangentially oriented relative to the side wall for inducing the fluid flow to rotate about the longitudinal axis, thereby imparting centrifugal force on the solid material. A gaseous fluid outlet extends upwardly from the upper end of the housing for providing egress of a first, or separated, portion of the gaseous fluid. An outlet for solids is formed within the housing and positioned proximate the lower end. An agitator assembly is positioned within the housing proximate the lower end. The agitator assembly includes a vertically extending, rotatably supported drive shaft coaxially disposed with the longitudinal axis. The agitator assembly furthermore comprises a number of fixed anti-rotation members extending from the inner side of the housing to a central part of the housing. Due to the presence of these fixed anti-rotation members and the viscous nature of aqueous slurries of lignocellulosic biomass, the apparatus disclosed in U.S. Pat. No. 6,428,591 B1 will not be suitable for processing lignocellulosic biomass because of clogging risks.
In production scale processing of lignocellulosic biomass, we have under certain conditions experienced negative complications from use of the hydrocyclone system described in WO 2009/147512 A2.
This hydrocyclone system produces a temperature gradient within the fluid volume contained by the system. Pretreated biomass is released from a pressurized reactor at temperatures in the order of 170 degrees C. or higher into a fluid volume within the hydrocyclone. Water jets infuse hot water into the system so as to break up clumps of pretreated biomass material and provide for a smooth, continuous output of fluidized pretreated biomass slurry. But the result is invariably that a temperature gradient occurs within the hydrocyclone, the water at the top of the vessel, closest to the outlet from a pressurized reactor, being considerably warmer than the water at the outlet from the hydrocyclone.
It has been found that when using a device disclosed in WO 2009/147512 A2 for transferring a discharging hydrothermally pretreated biomass from higher to lower pressure regions, an undesirably high percentage of C5-sugars, such as xylan species, originally present in the lignocellulosic material is inevitably lost into the liquid phase of slurry. Such a loss of C5 sugars accordingly implies a lower yield of ethanol in the case where a hydrothermally pretreated lignocellulosic solid material is intended for hydrolysis and subsequent fermentation into ethanol.
As used herein the term “xylan” is used generically to refer to all oligo and polymer lengths of xylose, which is the predominant C5 sugar originating from hemicellulose. However “xylan” as measured includes a contribution of monomeric soluble xylose such that soluble “xylan” includes “xylose.”
Furthermore, with the device disclosed in WO 2009/147512 A2 it has been found that concurrently to this loss of C5-sugars into the liquid phase of the slurry, a furfural built-up in the liquid phase appears. It is well-known that furfural acts as a fermentation inhibitor for certain specific fermentation microorganisms in an ethanol fermentation of C5- and C6 sugars. Even small amounts of furfural present in liquid traces of the fibrous lignocellulosic biomass may represent a considerably inhibiting effect for the sugar fermenting microorgansims.
Moreover, with the device disclosed in WO 2009/147512 A2, under certain conditions, a still layer accumulates on the top of the hydrocyclone, in warmer temperatures, while “clumps” of pretreated biomass sediment to the bottom of the reactor, in cooler temperatures. As a consequence of this inhomogeneous distribution of pretreated biomass within the temperature gradient established within the hydrocyclone, the biomass may experience an inhomogeneous additional “cooking” after the primary pressurized pretreatment is concluded. This in turn causes difficulties in subsequent processing steps. Furthermore, as a consequence of sedimenting clumps of pretreated material accumulating on the bottom and sides of the reactor, a significant incidence of “lignin charring” may occur, which introduces a tendency for “clogging” of filters and pumps used in subsequent processing steps.
Accordingly, there exists a need for improved devices and methods for discharging pretreated biomass from higher to lower pressure in which the above outlined problems are eliminated.